1. Field of the Invention
The present invention relates to a method of forming an electrical fuse (e-fuse) and a metal gate transistor, and more particularly to a method of integrating an e-fuse process into a high dielectric constant (high-k) material and metal gate (HK/MG) process.
2. Description of Related Art
Electrical fuses (e-fuses) are generally applied to redundancy circuits in the integrated circuits. Once a defect is detected in the circuit, these e-fuses are used to trim or replace the defective part(s) of the circuit. In a conventional memory structure, some fuses are fabricated in the top metal layer. If some memory cells, word lines or lead wires fail, the fuses may be used to disconnect the failed parts, and some redundant memory cells, redundant word lines or redundant lead wires may replace the failed parts.
In addition, some fuse designs can even provide the programming function. For example, each transistor of a memory array may be connected to a metal wire in a wafer, and the metal wire may include a programmable connecting component. After the semiconductor chips are fabricated in the wafer, signals may be inputted into the semiconductor chips to specialize the standard chips into various kinds of product chips, so as to reduce the research and development cost, and also the fabricating cost. For inputting data into the programmable read only memory (PROM), a high voltage may be applied to the connecting wire to burnout the programmable connecting component, so an open circuit (off-state) may be formed, and a digital signal “1” is inputted. On other hand, the un-burned fuse is connected to the transistor to form an on-state and a digital signal “0” is inputted. The procedure of blowing a fuse by a high voltage is called a programming step. Once the fuse is programmed, the fuse is rendered a permanently open circuit. Through the programming step, the programmed fuse and the un-programmed fuse serve as digital bits to store data.
As illustrated in FIG. 1, the prior art fuse unit 10 includes a poly-silicon e-fuse 12 and a controlling component 14, for example, is transistor. A terminal of the controlling component 14 is electrically connected to the poly-silicon e-fuse 12, and another terminal of the controlling component 14 is electrically connected to the ground point GND. In a normal condition, the fuse unit 10 is only a redundant part of the integrated circuit, and is not in use. While a trimming step or a programming step is performed, a gate voltage Vg is applied to the gate of the controlling component 14, and the controlling component 14 is therefore turned on. At this time, current Ids flows from the operating voltage Vfs through the poly-silicon e-fuse 12 to the ground point GND, and cause an electron migration in the poly-silicon e-fuse 12. When the current Ids continuously passes the poly-silicon e-fuse 12, the poly-silicon material of the poly-silicon e-fuse 12 moves along the boundaries of the crystalline grains, toward the current flow direction, and cause an open circuit for trimming or programming.
However, along with a trend towards scaling down the device size, the poly-silicon e-fuse is problematic in terms of device scaling. It is because the step of burning the poly-silicon e-fuse usually causes a particle pollution to damage the adjacent components. In order to reduce the potential damage to the adjacent components, large pitches between the poly-silicon e-fuse structure and the adjacent components are necessary, which decrease the component density. In addition, a sufficient current is necessary to burnout the poly-silicon e-fuse. Thus, a great voltage needs to be provided for such a programming step. Nevertheless, the voltage provided in the integrated circuit gets correspondingly smaller as the integrated circuit is being scaled down. Accordingly, control of the operating voltage for an e-fuse gets harder, and application of the poly-silicon e-fuse is limited. The present programming scheme may be inoperable due to the intrinsic robustness of the poly-silicon material. In the case of a highly reliable or compact circuit system, an on-off ratio of 2 to 3 orders of magnitude may hinder this usage range and applicability.
In light of this, the poly-silicon e-fuse structure limits increase in the component density. It is still a challenge to provide an e-fuse structure for nano-scaled integrated circuit.